Contemporary radiation therapy utilizes several types of ionizing radiation, such as .beta.-rays, .gamma.-rays, X-rays and high-energy protons applied to malignant tissue to prevent and control the spread of cancer. Proton beam therapy, in particular, has undergone dramatic development in recent years with attendant advances in therapy techniques and facilities. In most proton therapy systems around the world, the proton accelerators were originally built for physics research and later adapted for part-time clinical research and therapy. However, the imminent advantages of proton beam therapy is best realized with the development of dedicated, clinically based facilities. One such therapy facility, located at the Loma Linda University Medical Center, was purposely built to provide therapeutic proton beams to a multiplicity of treatment rooms, thereby increasing patient throughput and defraying the cost of an otherwise expensive form of treatment. An overview of the facility and its development is provided in "The Proton Treatment Center at Loma Linda University Medical Center: Rationale for and Description of its Development," J. M. Slater, et al., Intl. J. Radiation Oncology, vol. 22, no. 2, 1992, pp.383-389, and herein incorporated by reference. A more detailed description of the proton beam apparatus and facility is provided in U.S. Pat. No. 4,870,287 by F. T. Cole, et al., entitled "Multi-Station Proton Beam Therapy System," also herein incorporated by reference.
Proton radiation beamlines operate using large high-field electromagnets for beam deflection and focussing. At the Loma Linda Facility, the proton beam is generated with an on-site proton synchrotron and transferred by such beamlines to any one of several target destinations. To insure protection from proton radiation exposure, the beamline magnets must be monitored and controlled to prevent beam misdirection and mistiming. To this end, a method of treatment room selection verification has been employed by which a selected or desired beam path implementation is verified before authorizing beam delivery. The method of selection verification is more fully disclosed in U.S. Pat. No. 5,260,581, herein incorporated by reference. While such a method appears to be a necessary safeguard from radiation misexposure, it does not detect all potentially hazardous fault conditions. The collection of such magnets often require megawatts of electrical power, which itself presents a lethal hazard to facility personnel if appropriate safeguards are not taken to insure personnel non-contact. Thus, in addition to properly coordinating and timing the magnet array, appropriate measures should be taken to insure against mechanical, electrical and thermal breakdown. In the event of component failure, by any means, the high-power apparatus should be disabled and the radiation beam directed to a so-called "beam dump." Clearly, meeting these challenging demands is a necessity of great importance.
In the broader context of radiation therapy, a necessary precondition for treatment is the safeguard of patients and personnel from accidental radiation exposure. In particular, at proton beam treatment facilities, accidental exposure to beam radiation or derivatives thereof is perceived to be the principal threat to patient and personnel safety. Inadvertent exposure to radiation may for example occur through beam misdirection or improper timing of radiation beam delivery. Nevertheless, as the demand for proton beam therapy increases and treatment facilities become more complex, as for example at the Loma Linda University Medical Center, beamline safety becomes a premium and the challenge of insuring beamline safety is taken very seriously.